What you are asking does not make much sense.
First C# is not necessarily calling out any UI. It is a programming language. It can be used in conjunction with different UI stacks like Winforms, WPF/Silverlight. In comparison you could use VB to make the same UI elements.
With that maybe it becomes clear to you that C# is the processing logic. If you want to combine with some legacy java stuff you have that is possible but you will run into some issues.
Here is an article that might help you get going:
Using Java Classes in your .NET Application[
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If you provide more details on your intention maybe you can get better assistance.
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